ALMA constraints on the faint millimetre source number counts and their contribution to the cosmic infrared background

Abstract

We have analysed 18 ALMA continuum maps in Bands 6 and 7, with rms down to 7.8 μJy, to derive differential number counts down to 60 μJy and 100 μJy at λ = 1.3 mm and λ = 1.1 mm, respectively. Furthermore, the non-detection of faint sources in the deepest ALMA field enabled us to set tight upper limits on the number counts down to 30 μJy. This is a factor of four deeper than the currently most stringent upper limit. The area covered by the combined fields is 9.5 × 10^(-4) deg^2 at 1.1 mm and 6.6 × 10^(-4) deg^2 at 1.3 mm. With respect to previous works, we improved the source extraction method by requiring that the dimension of the detected sources be consistent with the beam size. This method enabled us to remove spurious detections that have plagued the purity of the catalogues in previous studies. We detected 50 faint sources (at fluxes <1 mJy) with signal-to-noise (S/N) >3.5 down to 60 μJy, hence improving the statistics by a factor of four relative to previous studies. The inferred differential number counts are dN/ d(Log_(10)S) = 1 × 10^5 deg^2 at a 1.1 mm flux S_λ = 1.1 mm = 130 μJy, and dN/ d(Log_(10)S) = 1.1 × 10^5 deg^2 at a 1.3 mm flux S_λ = 1.3 mm = 60 μJy. At the faintest flux limits probed by our data, i.e. 30 μJy and 40 μJy, we obtain upper limits on the differential number counts of dN/ d(Log_(10)S) < 7 × 10^5 deg^2 and dN/ d(Log_(10)S) < 3 × 10^5 deg^2, respectively. Determining the fraction of cosmic infrared background (CIB) resolved by the ALMA observations was hampered by the large uncertainties plaguing the CIB measurements (a factor of four in flux). However, our results provide a new lower limit to CIB intensity of 17.2 Jy deg^(-2) at 1.1 mm and of 12.9 Jy deg^(-2) at 1.3 mm. Moreover, the flattening of the integrated number counts at faint fluxes strongly suggests that we are probably close to the CIB intensity. Our data imply that galaxies with star formation rate (SFR) < 40 M_⊙/yr certainly contribute less than 50% to the CIB (and probably a much lower percentage) while more than 50% of the CIB must be produced by galaxies with SFR> 40 M_⊙/yr. The differential number counts are in nice agreement with recent semi-analytical models of galaxy formation even as low as our faint fluxes. Consequently, this supports the galaxy evolutionary scenarios and assumptions made in these models.

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